Water-soluble salts of nitrilotriacetic acid are desirable detergent builders in formulations without phosphate builders. The powder form of the material tends to cake and there is little absorptivity. Detergent builders which absorb liquids have become desirable in recent years as a formulating tool which allows surfactant to be incorporated into a detergent formulation by way of being absorbed into the builder component.
Few process are known for granulating alkali metal nitrilotriacetate (NTA) and none for increasing the absorptivity of NTA. One such process is described in U.S. Pat. No. 3,629,329. Previous attempts to granulate NTA were motivated by the desire to reduce the caking tendency of the product. For this purpose a reaction mixture of the acid precursor, an alkali metal carbonate and water was formed wherein molar ratio of NTA to the alkali metal content of the carbonate was present in the ratio from about 1:2 to about 1:20. Water was added in the range of from about 5% to about 35% based on the total weight of the mixture. After mixing the mixture was dried to about 8% by weight moisture. A similar process, employing nitrilotriacetic acid and NTA in a ratio of 1:2 to 1:10 is disclosed in U.S. Pat. No. 3,591,630.
A composite detergent granulated product containing from about 3% to about 20% NTA and an inorganic salt selected from the group consisting of sodium metasilicate, sodium carbonate, and sodium sulfate is disclosed in U.S. Pat. No. 3,901,831. In this process, aqueous NTA is added to an agitated bed of inorganic salt having an initial temperature of from about 300.degree. C. to about 550.degree. C. so as to cool the bed to below 300.degree. C. The bed is then dehydrated at a temperature of about 120.degree. C.
A process for producing granular products containing disodium nitrilotriacetate is described in U.S. Pat. No. 3,546,123 wherein various acids are employed to wet trisodium nitrilotriacetate followed by agglomeration of the moistened particles and then drying the agglomerated particles to produce a product having a particle size in the range of smaller than a 4 mesh screen and larger than an 80 mesh screen and a density of from about 0.4 g/cc to about 0.8 g/cc. While such material is easily blended with spray dried detergent formulations, modern detergent formulations have a different consistency calling for a builder having a narrower range of density, i.e., above about 0.7 g/cc to be compatible with the remainder of the materials which may not be spray dried.
While flowability is improved, the above noted processes provided compositions having a wide range of particle size and densities, some of which provide little real difference between the granulated material and the powder material. Further, there is now a need for a nitrilotriacetate having considerably higher surfactant absorptivity than was achievable with prior art processes.
There is therefore needed a convenient process for the production of granular NTA having a consistent density in the medium range, that is, from about 0.7 g/cc to about 0.81 g/cc. Because the powder form of NTA is practically non-absorptive (about 2 g/cc) there is a need to produce NTA having a greatly increased absorptivity capacity in the range of from about 6 to 7 times greater than is now commercially available.